Avicenna, also known as Ibn Sînâ, is the most important philosopher in the Arabic tradition, and one of the world’s greatest thinkers. Like his predecessors, al-Kindî and al-Fârâbî, and his successor, Averroes, Avicenna self-consciously marked himself out as a philosopher rather than an Islamic theologian, choosing to follow Greek wisdom and the path of reasoning and proof. In particular, he saw himself as a follower of Aristotle, and his main writings are encyclopedias of Aristotelian philosophy.

Aristotle claims that the body and mind of humans (and other animals) are not two different things (or “substances”), but one unit, and that the mind is the “form” of the human body. As such, it is responsible for all the activities a human being can perform, including thinking. For this reason Aristotle does not seem to think it possible for anything to survive the death of the body.

By contrast, Avicenna is one of the most famous “dualists” in the history of philosophy—he thinks that the body and the mind are two distinct substances. His great predecessor in this view was Plato, who thought of the mind as a distinct thing that was imprisoned in the body. Plato believed that at the point of death, the mind would be released from its prison, to be later reincarnated in another body.

In seeking to prove the divided nature of mind and body, Avicenna devised a thought-experiment known as the “Flying Man”. This appears as a treatise, On the Soul, within his Book of Healing, and it aims to strip away any knowledge that can possibly be disproved, and leave us only with absolute truths.

It remarkably anticipates the much later work of Descartes, the famous dualist of the 17th century, who also decided to believe nothing at all except that which he himself could know for certain. Both Avicenna and Descartes want to demonstrate that the mind or self exists because it knows it exists; and that it is distinct from the human body.

The Flying Man

In the Flying Man experiment, Avicenna wants to examine what we can know if we are effectively robbed of our senses, and cannot depend on them for information. He asks us each to imagine this: suppose I have just come into existence, but I have all my normal intelligence. Suppose, too, that I am blindfolded and that I am floating in the air, and my limbs are separated from each other, so I can touch nothing. Suppose I am entirely without any sensations. None the less, I will be sure that I myself exist. But what is this self, which is me? It cannot be any of the parts of my body, because I do not know that I have any. The self that I affirm as existing does not have length or breadth or depth. It has no extension, or physicality. And, if I were able to imagine, for instance, a hand, I would not think that it belonged to this self which I know exists. It follows from this that the human self—what I am—is distinct from my body, or anything physical. The Flying Man experiment, says Avicenna, is a way of alerting and reminding oneself of the existence of the mind as something other than, and distinct from, the body.

Avicenna goes on to draw the conclusion that the mind is not destroyed when the body dies, and that it is immortal.

Ghost in the Machine

One very strong objection to the dualism of Avicenna or Descartes is the argument used by Aquinas. He says that the self which thinks is the same as the self which feels sensations in the body. For instance, I do not just observe that there is a pain in my leg, in the way that a sailor might notice a hole in his ship. The pain belongs to me as much as my thoughts about philosophy, or what I might have for lunch.

Most contemporary philosophers reject mind-body dualism, largely because of the increasing scientific knowledge of the brain.

Avicenna and Descartes were both very interested in physiology and they produced scientific accounts of activities such as movement and sensation. But the process of rational thinking was inexplicable with the scientific tools of their times. We are now able to explain quite precisely how thinking goes on in different areas of the brain—though whether this means that we can explain thinking without reference to a self is not so clear. An influential 20th-century British philosopher, Gilbert Ryle, caricatured the dualists’ self as “a ghost in the machine”, and tried to show that we can explain how human beings perceive and function within the world without resorting to this “ghost” of a self.

Today philosophers are divided between a small number of dualists, a larger number of thinkers who say that the mind is simply a brain, and the majority, who agree that thinking is the result of the physical activity of the brain, but still insist there is a distinction between the physical states of the brain (the gray matter, the neurons, and so on), and the thinking which derives from them.

Many philosophers, especially continental European thinkers, still accept the results of Avicenna’s thought experiment in one central way. It shows, they say, that we each have a self with a first-person view of the world (the “I”) that cannot be accommodated by the objective view of scientific theories.